Radio frequency identification functionality coupled to electrically conductive signage

ABSTRACT

The present disclosure relates to multiple embodiments of a signage having radio-frequency responsive features, methods of making and using the signage, and the performance characteristics of the signage. These embodiments include a cutout, aperture, or opening in an electrically conductive sign into which or adjacent to which is placed an RFID tag or chip.

RELATED APPLICATION

This application claims the benefit under 35 U.S.C. § 119(e) of U.S.Provisional Application No. 60/925,103 filed Apr. 18, 2007.

TECHNICAL FIELD

The present disclosure relates to multiple embodiments of a signagearticle having radio-frequency responsive features, methods of makingand using the signage article, and the performance characteristics ofthe signage article.

BACKGROUND

Radio frequency identification technology, sometimes referred to as RFIDtechnology, has a variety of commercial applications, and is typicallyused for object identification and tracking.

This section describes at least one embodiment of a typical radiofrequency identification (“RFID”) tag and reader, this embodiment andothers are well known in the art. FIG. 1 illustrates a typical radiofrequency identification (“RFID”) tag 10. The RFID tag 10 includes asubstrate 12 having a first major surface 14 and a second major surface16 opposite the first major surface 14. The substrate 12 may optionallybe a flexible substrate, such that it could be used in a label that maybe wrapped around an object. The flexible substrate 12 could have enoughflexibility to conform to a variety of surfaces and bend easily aroundobjects. For example, the substrate 12 may be in the range of 25-100microns in thickness, and may be made of a flexible material, such aspolyester, polyethylene naphthanate, polyimide, polypropylene, paper, orother flexible materials apparent to those skilled in the art.

An RFID element is attached to the first major surface 14 of thesubstrate 12. The RFID element has information storage and transmissioncapabilities adapted to enable an interrogation system to obtaininformation from the radio frequency-responsive element. The RFIDelement typically includes two major components: an integrated circuit20 and an antenna 18. The integrated circuit 20 provides the primaryidentification function. It includes circuitry to permanently store thetag identification and other desirable information, interpret andprocess commands received from the interrogation hardware, respond torequests for information by the interrogator, and assist the hardware inresolving conflicts resulting from multiple tags responding tointerrogation simultaneously. Optionally, the integrated circuit mayprovide for updating the information stored in its memory (read/write)as opposed to just reading the information out (read only). Someexemplary integrated circuits suitable for use in RFID tags 10 includethose commercially available from Texas Instruments™ (in their line ofproducts under the trade names TI-RFid™ or TAG-IT™), Philips and/or NXPElectronics Co. (in their line of products under the trade namesI-CODE™, MIFARE™, and HITAG™), among others.

The antenna 18 geometry and properties depend on the desired operatingfrequency of the RFID tag 20. For example, 915 MHz or 2.45 GHz RFID tags10 would typically include a dipole antenna, such as a linear dipoleantenna or a folded dipole antenna (not shown). A 13.56 MHz (or similar)RFID tag 10 would typically use a spiral or coil antenna 18, as shown inFIG. 1. However, other antenna designs are known to those skilled in theart. The antenna 18 intercepts the radio frequency energy radiated by aninterrogation source, such as a RFID reader 60 illustrated schematicallyin FIG. 2. (Reference number 62 illustrates the radio frequency energyradiated by the RFID reader 60.) Radio frequency energy 62 carries bothpower and commands to the tag 10. The antenna enables the RF-responsiveelement to absorb energy sufficient to power the integrated circuit 20and thereby provide the response to be detected. Thus, thecharacteristics of the antenna are typically matched to the system inwhich it is incorporated. In the case of tags operating in the high MHzto GHz range, one of the most important characteristics is typically theantenna size. Often, the effective length of a dipole antenna isselected so that it is close to a half wavelength or multiple halfwavelength of the interrogation signal. In the case of tags operating inthe low to mid MHz region (13.56 MHz, for example) where a halfwavelength antenna is impractical due to size limitations, the importantcharacteristics are typically antenna inductance and the number of turnson the antenna coil. Often, metals such as copper or aluminum are used,but other conductors, including printed inks, are also acceptable. It isalso important that the input impedance of the selected integratedcircuit match the impedance of the antenna for maximum energy transfer.Additional information about antennas is known to those of ordinaryskill in the art, for example, in reference texts such as RFID Handbook,Radio-Frequency Identification Fundamentals and Applications, by K.Finkenzeller, (1999 John Wiley & Sons Ltd, Chichester, West Sussex,England).

A capacitor 22 is often included to increase the performance of the RFIDtag 10. The capacitor 22, when present, aids in tuning the operatingfrequency of the tag to a particular value. This is desirable forobtaining maximum operating range. The capacitor may either be adiscrete component or may be integrated into the antenna 18.

An RFID reader or interrogator 60 is schematically illustrated in FIG.2. The RFID reader 60 includes an RFID reader antenna 64. RFID readers60 are well known in the art. For example, commercially available RFIDreaders are available from 3M Company based in St. Paul sold under thetrade name 3M™ Digital Library Assistant™ as model numbers 702, 703,802, and 803. Another example of a commercially available RFID reader isa model IP4 portable RFID (UHF) Reader attached to an Intermec™ 700Series Mobile computer available from Intermec Technologies Corporation,Everett, Wash.

The RFID reader 60 and RFID tag 10 form an RFID system. Inductivelycoupled RFID systems are based on near-field magnetic coupling betweenthe antenna loop of the RFID reader and the antenna coil of the RFIDtransponder, according to RFID Handbook, Radio-Frequency IdentificationFundamentals and Applications, by K. Finkenzeller, (1999 John Wiley &Sons Ltd, Chichester, West Sussex, England) pp. 21. A number of RFIDsystems are available, following one of several communication and systemperformance standards. The discussion below is principally based on RFIDsystems operating at 13.56 MHz, but the discussion extends toinductively coupled RFID systems at other operating frequencies andprovides insights into the interference that conductive objects can poseto electromagnetically coupled RFID systems.

Radio frequency-responsive tags can be either active or passive. Anactive tag incorporates an additional energy source, such as a battery,into the tag construction. This energy source permits active radiofrequency-responsive tags to create and transmit strong response signalseven in regions where the interrogating radio frequency field is weak,and thus an active radio frequency-responsive tag can be detected atgreater range. However, the relatively short lifetime of the batterylimits the useful life of the tag. In addition, the battery adds to thesize and cost of the tag. A passive tag derives the energy needed topower the tag from the interrogating radio frequency field, and usesthat energy to transmit response codes by modulating the impedance theantenna presents to the interrogating field, thereby modulating thesignal reflected back to the reader antenna. Thus, their range is morelimited. Because passive tags are preferred for many applications, theremainder of the discussion will be confined to this class of tag. Thoseskilled in the art, however, will recognize that these two types of tagsshare many features and that both can be used in the examples of thisdisclosure.

SUMMARY

The inventors of the present application recognized that by couplingRFID functionality to a signage, such as, for example, a metal sign, thesignage can be associated with data stored in the RFID tag or chip thatis affixed to or integrated with the metal sign. The prior art attemptsto attach or couple an RFID tag to a metal sign have certain drawbacks.However, the inventors of the present application discovered variousways to successfully attach or couple RFID functionality to anelectrically conductive signage.

One method of attaching or coupling RFID functionality to a signageinvolves physically coupling a functioning RFID tag and an electricallyconductive signage. The resulting signage includes a functioning RFIDtag that is inset into the signage.

An alternative method of attaching or coupling RFID functionality to asignage involves forming a slot, opening, or aperture in the signage andusing that slot, opening, or aperture to act as an antenna for an RFIDchip that is physically coupled to the signage.

One exemplary embodiment of an electrically conductive RFID-enabledsignage includes an electrically conductive element including a cutoutand an RFID tag inset into the cutout. The electrically conductiveelement includes at least one of an electrically conductive substrate oran electrically conductive sheeting. For example, the electricallyconductive element may be a metal substrate positioned adjacent tononconductive or conductive optically active sheeting, such as, forexample, retroreflective or reflective sheeting. Alternatively, theelectrically conductive element may be a nonconductive substratepositioned adjacent to conductive optically active sheeting, such as,for example, retroreflective or reflective sheeting. Thus the electricalconductivity of the signage article may be derived from the use of anelectrically conductive signage substrate or from the use of anelectrically conductive sheeting positioned adjacent to the signagesubstrate.

An exemplary method of forming an electrically conductive, RFID-enabledsignage article involves selecting a location on an electricallyconductive element; forming a cutout in the electrically conductiveelement at the selected location; and placing a radio frequencyidentification (RFID) tag into the cutout. The location selection stepis based on consideration of a desired radiation pattern of theelectrically conductive, RFID-enabled signage article that is formedwhen the RFID tag is placed into the cutout.

Another exemplary method involves forming a cutout having a cutout sizeand a cutout shape in an electrically conductive element having aconductive portion size and a conductive portion shape and placing aradio frequency identification (RFID) tag into the cutout to form anelectrically conductive, RFID-enabled signage article. In someembodiments, the selection of the cutout size may be based on a desiredradiation pattern. In some embodiments, the selection of the cutoutshape may be based on a desired radiation pattern. In some embodiments,the selection of the conductive portion size may be based on a desiredradiation pattern. In some embodiments, the selection of the conductiveportion shape may be based on a desired radiation pattern.

Another exemplary embodiment of an electrically conductive, radiofrequency identification (RFID)-enabled signage article includes a slotantenna including an electrically conductive element having an opening;and an RFID integrated circuit coupled to the substrate. The slotantenna operates as an RFID antenna, and the electrically conductiveelement includes at least one of an electrically conductive substrate oran electrically conductive sheeting.

One method of forming an electrically conductive, radio frequencyidentification (RFID)-enabled signage article involves selecting anelectrically conductive element having a length and a width; forming anopening in the electrically conductive element; and coupling a radiofrequency identification (RFID) integrated circuit to at least a portionof the electrically conductive element. The step relating to selectingan electrically conductive element having a length and a width may bebased on a desired radiation direction and/or pattern of a electricallyconductive, RFID-enabled signage article that is formed when the RFIDintegrated circuit is coupled with the electrically conductive element.

Another method involves selecting a location on an electricallyconductive element; forming an opening in at least a portion of theelectrically conductive element at the selected location; and coupling aradio frequency identification (RFID) integrated circuit to at least aportion of the electrically conductive element. The selecting a locationon the electrically conductive element step may be based on a desiredradiation direction and/or pattern of the electrically conductive,RFID-enabled signage article that is formed when the RFID integratedcircuit is coupled with the electrically conductive element.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a top view of a radio frequency identifications (“RFID”) tagknown in the art.

FIG. 2 is a schematic view of interactions between the RFID tag of FIG.1 and a RFID reader.

FIG. 3 illustrates the interaction between the RFID tag of FIG. 1 and aconductive object.

FIG. 4 illustrates the interaction between the RFID tag and conductiveobject of FIG. 3 and a spacer.

FIG. 5 a is a schematic view of one embodiment of a traffic signagearticle used as an RFID tag antenna.

FIG. 5 b is an exploded view of the RFID tag of FIG. 5 a.

FIG. 6 a is a schematic view of another embodiment of a traffic signageincluding an RFID tag.

FIG. 6 b is an exploded side view of the RFID tag area of FIG. 6 a.

FIG. 7 is a pictorial depiction of the location of multiple cutouts thatwere cut from various plates for testing the effects of the cutoutlocation.

FIGS. 8A, 8B, 8C, and 8D are pictorial depictions of how the thirteencutout locations shown in FIG. 7 were implemented in four separatepieces of metal plate.

FIG. 9 is a graph that shows the effective radiation pattern of an RFIDtag in cutouts A through I.

FIG. 10 is a graph that shows the effective radiation pattern of an RFIDtag in cutouts 1 through 5.

FIG. 11 is a graph that shows the effect of having an RFID tag centeredalong the x-axis and y-axis of a cutout but at different locations alongthe z-axis of the cutout.

FIG. 12 is a modeled radiation profile relating to the sample describedin Example 3.

FIG. 13 is a modeled radiation profile relating to the sample describedin Example 4.

FIG. 14 is a modeled radiation profile relating to the sample describedin Example 5.

FIG. 15 is a schematic view of a slot design.

FIGS. 16 a and 16 b shows the respective front and side view modeledradiation profile of the slot antenna of FIG. 15.

FIGS. 17 a and 17 b show the respective front and side view antennapattern for Plate #2 of Example 7.

FIGS. 18 a and 18 b show the respective front and side view antennapattern for Plate #3 of Example 7.

FIG. 19 is a comparison of the E-plane (y-z plane) antenna patternsshown in FIGS. 16-18.

FIGS. 20 and 21 are schematic diagrams showing the locations of variousslots that were modeled in a plate.

FIG. 22 shows the modeling results for the E-plane (y-z plane) thatrelate to the slot locations shown in FIG. 20.

FIG. 23 shows the modeling coordinate system.

FIG. 24 shows the H-plane (x-y plane) modeling results that relate tothe slot locations shown in FIG. 20.

FIGS. 25 a and 25 b are schematic views of a flush mounted integratedtag with a matching circuit.

FIG. 26 is an electrical circuit diagram of the matching circuit shownin FIG. 25.

DETAILED DESCRIPTION

As used herein, the term “RFID label” is used interchangeably with theterm “RFID tag.” In addition, the term “RFID element” may refer to aRFID label or tag as well as to any component that makes up an RFIDlabel or tag (e.g., an RFID integrated circuit, antenna or electricalinterconnect network).

The inventors of the present patent application recognized that it wouldbe beneficial to attach an RFID label to a signage in order to, forexample, facilitate identification and tracking of the signage and toimprove maintenance and replacement services. The RFID labels maycontain information stored on the integrated chip related to the part itis attached to, for example, signage type, date of manufacture,manufacturer codes, date of installation and maintenance, signagelocation and position, signage material, signage constructioninformation, reflectance or other performance measurements, date of lastinspection, and inspection information. This information can beparticularly useful in the maintenance of the signage because theinspection history of the signage can be stored on the RFID label as itgoes through different stages of its life cycle. Additionally, thisinformation can be useful to electronically provide information toroadway vehicles, one example being as an automatic navigational aid.

However, many types of signage, including metal or otherwiseelectrically conductive signage, and typical RFID elements generally donot work properly when operated within about a quarter inch (0.64 cm),or 6 mm (15.24 cm), of one another. In short, when the RFID tags orlabels are in close proximity to a conductive object, such as a metalroad sign, there tends to be interference problems resulting in the RFIDreader being unable to successfully read the RFID tag.

This section describes the typical interactions between RFID tags andRFID readers, and the interference problems typically encountered whenRFID tags are in close proximity to electrically conductive objects.FIG. 2 illustrates the RFID reader 60 interrogating an RFID tag 10 thatis not located close to an electrically conductive object. FIG. 3illustrates the interrogation of an RFID tag 10 in close proximity to anelectrically conductive object 24. Examples of electrically conductiveobjects 24 include objects containing metal, nonmetallic substances(e.g., carbon-fiber based composite), or liquid (e.g., an aqueous ionicsolution in a bottle). For example, an electrically conductive objectcould include a metal car part or tool. FIG. 4 illustrates theinterrogation of the RFID tag 10 in close proximity to the electricallyconductive object 24 with a prior art spacer layer 66 located betweenthe RFID tag 10 and the electrically conductive object 24.

As illustrated in FIG. 2, when the RFID reader 60 is attempting tointerrogate the RFID tag 10, the RFID reader 60 produces a time-varyingelectrical current in the RFID reader antenna 64. The variations inelectrical current may be a smoothly varying sinusoidal carrierfrequency, or the variations may be aperiodic and non-repetitivevariations in amplitude, frequency, or phase of the sinusoidal carrierfrequency representing encoded digital data. The time-varying electricalcurrent produces an electromagnetic field, which extends through spaceto the RFID antenna 18. The time-varying magnetic flux through the RFIDantenna 18 induces an electromotive force (EMF) in the RFID antenna 18,according to Faraday's Law of Induction, which is described in moredetail in Electromagnetism by John C. Slater and Nathaniel H. Frank,(1969 Dover Publications, New York), pp. 78-80. The induced EMF appearsas an effective induced voltage across the two end terminals of the RFIDantenna 18, hence giving the classification known in the art as an“inductively coupled RFID system.” The induced voltage drives atime-varying electrical current through the RFID integrated circuit 20,thereby completing the RFID communication link from the RFID reader 60to the RFID tag 10.

When, as illustrated in FIG. 3, the RFID antenna 18 is not in freespace, but is adjacent to an item with finite electrical conductivity,such as an electrically conductive object 24, the EMF induced in theRFID transponder antenna is reduced, generally to a level at which thetag is not able to respond. This occurs when situations such as thatillustrated by FIG. 3 occur, i.e., when the plane of the RFID antenna 18is substantially parallel with and proximate to the surface of theelectrically conductive object 24. This might be the case if, forexample, the RFID tag 10 is attached to the electrically conductiveobject 24 as a label to identify the object. According to Faraday's Lawof Induction, eddy currents 23 will be induced in the conductive object,as is discussed in more detail in Electromagnetism by John C. Slater andNathaniel H. Frank, (1969 Dover Publications, New York) pp. 78-80.According to Lenz's Law, the net effect of the eddy currents 23 isreduction of the magnetic flux near the conductive object, as isdiscussed in more detail in the RFID Handbook, Radio-FrequencyIdentification Fundamentals and Applications, by K. Finkenzeller, (1999John Wiley & Sons Ltd, Chichester, West Sussex, England) p. 64. Thereduced net magnetic flux near the electrically conductive objectresults in a reduced EMF in the RFID transponder antenna, compared tothe first case illustrated in FIG. 2, where the RFID antenna 18 was infree space.

If the antenna 18 of the RFID tag 10 is a rectilinear antenna, asillustrated in FIG. 4, then the conductors that comprise the antenna 18are essentially long straight conductors, connected at each end toadjacent conductors to form a loosely coiled antenna form. Theelectrical current I in each long straight portion of each conductor inthe RFID antenna 18 sets up a magnetic field H at a distance r away fromeach portion respectively, according to the following formula:

H=I/(2πr)

If the RFID tag 10 is proximate or adjacent to the electricallyconductive object 24, the magnetic fields generated by each conductorsegment will induce a counter-circulating eddy current 23 in theelectrically conductive object 24, as illustrated by the clockwisearrow. The strength of the induced eddy current 23 depends on the amountof magnetic field energy coupled into the conductive substrate. If theRFID tag 10 is attached to the electrically conductive object 10, forexample by a thin layer of adhesive, the energy coupled from the RFIDtag 10 to the electrically conductive object 24 will be large and theinduced eddy current 23 will be correspondingly large. If the eddycurrent 23 is similar in magnitude to the RFID tag 10 current, butopposite in direction, the sum of the transponder current and the eddycurrent will be essentially zero and the RFID tag 10 will not bedetected by the RFID reader 60. This physical phenomenon is oftenreferred to by those skilled in the art as “interference problems” whenRFID tags are in close proximity with electrically conductive objectssuch as metal objects.

Various attempts have been made to reduce or eliminate the interferenceproblems described above when an RFID tag is proximate or adjacent to anelectrically conductive object. Using some of these methods, as isdiscussed in greater detail below, it is possible for an RFID reader toproperly read the RFID tag, despite its location next to theelectrically conductive object. Various methods described in the priorart literature may be used to electromagnetically decouple the RFIDtransponder from the electrically conductive surface. Examples of suchmethods are disclosed in the following publications and patent: PCTPublication WO 03/030093 (Gschwindt), “Transponder Label and Method forthe Production Thereof,” PCT Publication WO 03/067512 (Surkau),“Transponder Label”; and U.S. Pat. No. 6,371,380 (Tanimura),“Non-Contacting-Type Information Storing Device. WO 03/030093 describesa shielding layer that has ferrite particles embedded therein. WO03/067512 also describes a shielding film that has ferrite particlesembedded therein. Ferrite particles are inorganic compounds containingiron in one of its natural oxidation states (Fe3+) chemically bound withoxygen and other chemical elements. Typically, the ferrite particles areuniform in composition throughout the particle, and homogenous, forexample, the ferrite compound is the same throughout the full depth ofthe particle. U.S. Pat. No. 6,371,380 describes using a magnetismabsorbing plate formed from Sendust. Although not stated in the '380patent, it is known in the industry that Sendust is made from a ferrousalloy powder. The base material is approximately 85% iron, 6% aluminum,and 9% silicon. (See, for example, Soft Magnetics Application Guidepublished by Arnold Magnetic Technologies Corporation, Rochester, N.Y.,p. 30-1, February 2003 Rev. B.).

Another solution taught in the prior art to help reduce the interferenceproblems is to insert a nonconductive, nonmagnetic dielectric physicalspacer, for example, polymer film, foam tape, or similar materialsbetween the conductive object 24 and the RFID tag 10. The physicalspacer increases the distance between the conductors comprising the RFIDantenna 18 and the substrate of the electrically conductive object 24.According to the equation referenced below,

H=I/(2πr),

when the distance r between the RFID antenna 18 and the substrate of theelectrically conductive object 24 is increased, the magnetic fieldintensity H is commensurately reduced at the surface of the electricallyconductive object. In this condition, the magnetic field energy coupledto the electrically conductive object is reduced, compared to the casewhere the RFID tag is directly adjacent the electrically conductiveobject 24. However, again, the disadvantage of this approach is theadditional thickness that is required by the polymer film, foam tape orother similar materials to put adequate distance between the RFID tagand the electrically conductive object to help reduce or eliminate theinterference problems. The examples illustrate the typical thicknessesof nonconductive, nonmagnetic, dielectric physical spacers, such as foamcore, paper, or polymer films, which are required to successful read anRFID tag adjacent an electrically conductive surface with an RFIDreader.

Each of these methods have certain disadvantages. The present inventorsrecognized a need to provide various alternative methods of reducing oreliminating interference problems when an RFID tag or chip is attachedto an electrically conductive signage. The present inventors alsorecognized the benefit of associating RFID functionality with a signage,including an electrically conductive signage, such as a metal signage.Some preferred solutions have a relatively low areal mass density andtherefore have relatively minimal impact to the mass of the overallsignage. In addition, because multiple signages may be stacked in a pilebefore and during installation, the present inventors also recognizedthat some preferred embodiments might comprise an RFID-enabled signagecapable of being stacked with other RFID-enabled signages withoutdamaging the RFID elements in any of the RFID-enabled signages. In someembodiments, the RFID elements are placed within the signage thicknessor are flush with a major signage surface. Additionally oralternatively, the present inventors also recognized that some preferredembodiments may include RFID elements that are permanently attached oraffixed to the signage and some preferred embodiments may include RFIDelements that are removably attached or affixed to the signage.

The present inventors invented multiple constructions, methods of makingand using, and embodiments of an electrically conductive signage havingRFID functionality. One exemplary construction and/or embodimentinvolves attaching or coupling RFID functionality to a signage byphysically inseting a functioning RFID tag into an electricallyconductive signage. An alternative exemplary construction and/orembodiment involves attaching or coupling RFID functionality to asignage by forming a slot, opening, or aperture in the signage and usingthat slot, opening, or aperture to act as an antenna for an RFID chipthat is physically coupled to the signage. The resulting signageincludes an RFID integrated circuit that is attached to the signage suchthat the slot creates an antenna for the RFID integrated circuit. Theseconstructions, embodiments, and methods are discussed in greater detailbelow.

Exemplary signage articles include, but are not limited to, trafficcontrol materials; retroreflective, non-retroreflective, reflective, andnon-reflective vehicle or roadway markings; retroreflective garments;indoor/outdoor labeling products; frangible security stickers; productauthentication materials; store display packages; documents; inventorylabeling and control products; identification tags, labels, or systems;and license plates.

I. Signage Constructions Including a Functioning RFID Tag and anElectrically Conductive Substrate

One exemplary embodiment of the present application is shown in FIGS. 6a and 6 b. FIG. 6 a shows a schematic view of an electricallyconductive, RFID-enabled signage article 100. Signage article 100includes a signage substrate 101 into which has been formed a cutout,opening, or aperture 108. Cutout 108 is shown as a rectangular cutout inFIGS. 6 a and 6 b, but can be any shaped cutout. FIG. 6 b is anexploded, side view of signage article 100. FIG. 6 b shows a first majorsurface 104 and a second major surface 106 of signage substrate 101. Alayer of optically active sheeting 102 (including, but not limited to,retroreflective or reflective sheeting) is positioned adjacent to atleast a portion of first major surface 104 of sign substrate 101.Sheeting 102 may be directly in contact with signage substrate 101.Sheeting 102 may alternatively be adjacent to signage substrate 101,such as, for example, when a layer of adhesive holds sheeting 102adjacent to sign substrate 101.

In FIGS. 6 a and 6 b, the combination of the substrate and the sheetingform an electrically conductive element. As described in greater detailbelow, many options for an electrically conductive element exist. Anelectrically conductive element may include substrate 101 and sheeting102 or may include only one of substrate 101 or sheeting 102. Further,substrate 101 may be electrically conductive or electricallynonconductive and sheeting 102 may be electrically conductive orelectrically nonconductive. For example, substrate 101 may beelectrically conductive and sheeting 102 may be electricallynonconductive; substrate 101 may be electrically nonconductive andsheeting 102 may be electrically conductive, or both substrate 101 andsheeting 102 may be electrically conductive. Further, cutout 108 may beformed in either or both of substrate 101 and sheeting 102. For example,cutout 108 may be formed in both substrate 101 and sheeting 102; in onlysubstrate 101; or in only sheeting 102. Cutout 108, however, maypreferably be formed in at least the electrically conductive portions ofan electrically conductive element.

A fully functioning RFID tag 110 (e.g., an integrated circuit, antenna,and any electrical interconnect network) is inset into cutout 108. InFIG. 6 b, RFID tag 110 is positioned in cutout such that a first majorsurface 105 of RFID tag 110 is adjacent to sheeting 102. A plug 112 ispositioned within cutout 108 such that it is adjacent to a second majorsurface 107 of RFID tag 110. A layer of tape 114 holds plug 112 and RFIDtag 110 within cutout 108.

In the embodiment shown in FIGS. 6 a and 6 b, RFID tag 110 (includingits antenna) is relatively flush with signage substrate 101 and tape 114preferably is of minimal thickness (for example, a thickness betweenabout 0.5 mil to about 5 mil) such that no part of RFID tag 110significantly protrudes from the surfaces 104, 106 of signage article100 to an extent that would prohibit multiple signs from being stackedupon one another.

Signage substrate 101 may be formed of an electrically conductivematerial or of a nonconductive material. Exemplary electricallyconductive materials include, for example, a metal plate, such as, forexample, an aluminum plate. Exemplary nonconductive materials include,for example, wood or plastic. Where signage substrate 101 comprises anonconductive material, electrical conductivity of the completeelectrically conductive, RFID-enabled signage article may come from, forexample, an electrically conductive sheeting placed on at least aportion of the signage article, such as, for example, metallizedretroreflective sheeting. For purposes of this application, sheetingwith a metal (e.g., aluminum) vapor coat is considered conductive,although the level of conductivity may be minimal. Exemplary metallizedsheetings include, for example, the following commercially availableproducts the 3290T series of sheeting products; the CW80 series ofsheeting products; high intensity beaded sheeting, such as, for example,the 3870 series of sheeting products; flexible high intensity sheeting,such as, for example, the 3810 series, the 3840, and the 31x barricadesheeting products; and license plate or validation sheeting, all ofwhich are manufactured by 3M Company located in St. Paul, Minn.Additionally, any prismatic sheeting products including a vapor coatwould be considered conductive sheeting, such as, for example, the 985conspicuity sheeting manufactured by 3M Company. Exemplary nonconductivesheeting products include, but are not limited to, prismatic sheetingproducts that are not vapor coated such as, for example, the HIP™ 3930series, DG³™ 4000 series, VIP™ 3900 series, conspicuity 983 series, 3910series CWZ™ prismatic, and rollup signs RS20 and RS30 series, allmanufactured by 3M Company.

Those of skill in the art will appreciate that many changes can be madeto the implementation shown in FIGS. 6 a and 6 b without departing fromthe spirit of the concept. For example, although RFID tag 110 is shownpositioned adjacent to sheeting 102, plug 112 can be adjacent tosheeting 102 and RFID tag 110 can be adjacent to tape 114.Alternatively, signage article 100 can include two plugs, one of whichis adjacent to sheeting 102 and one of which is adjacent to tape 114 andeach of which are adjacent to a different major surface of RFID tag 110.Placement of the RFID tag 110 in relation to the thickness of signagesubstrate 101 may affect the tag performance, as will be discussed ingreater detail below.

Plug 112 can be formed of any suitable material, including, but notlimited to, any non-interfering, non-conductive material such as, forexample, plastic. Also, the cutout 108 formed in signage article 100need not be rectangular in shape; the cutout, opening, or aperture canbe of any desirable shape or size and can be placed in any desirablelocation on signage article 100. It will be noted that the shape andsize of the cutout may affect the performance of RFID tag 110 and mayincrease or decrease the amount of interference RFID tag 110experiences. Further, signage article 100 need not be a rectangularspeed limit sign, but can be any shape or form of signage. The tape 114that is used in this implementation may not only hold plug 112 and RFIDtag 110 in the cutout, but may also provide a weather-resistant sealthat protects RFID tag 110. Tape 114 may also have a barcode printed onit. A suitable tape 114 may be chosen to fulfill all of these interests.Also, plug 112 could be designed such that tape 114 is not required,thereby creating a completely flush design.

In another exemplary embodiment, a single signage article may includemore than one RFID tag. For example, using the technique described inExample 2, a first RFID tag could be placed such that it radiates moststrongly toward the front of the signage article, and a second RFID tagcould be placed such that it radiates most strongly toward the rear ofthe signage article. Further, the inclusion of a cutout in the signagearticle facilitates the formation of a signage article that can readvery well at an off-angle as well as from the front and rear of thesignage article.

Also, it will be appreciated that the above embodiments andimplementations may include a surface acoustic wave (SAW) RFID tagrather than a traditional RFID tag.

In at least some embodiments, metal in the electrically conductive sign(e.g., use of a metal signage substrate or use of metalized sheeting)facilitates the readability of the RFID tag. Thus the electricallyconductive sheeting and/or signage substrate assists in the performanceof the RFID tag rather than hindering its performance. Consequently, theread-range performance (at least over some range of angles) is enhancedin at least some embodiments. In addition to higher directivity, anotherperformance characteristic is the ability to at least partially restrictthe range of angles over which the tag can be read.

When additional control of the radiation pattern is desired (i.e.,control of nulls and/or beam lobes), it can be useful to includeadditional slots in the conductive sheeting to act as additional beamforming elements. These slots could be used as passive antenna elementsto create a beam forming array, such as in a yagi or other multi-elementantenna array design. For example, the elements can be arranged in anarray of elements to increase the antenna directivity and possiblycontrol the angle of maximum radiation. An array of elements includes,for example, multiple slots or insets that are electrically coordinatedby spacing and/or phasing to produce a desired radiation pattern.

One advantage of the use of the RFID antennas described above is theability to generate performance characteristics that are difficult toobtain from traditional RFID tag antennas on metal objects. Oneexemplary performance characteristic is high directivity, whichtranslates into longer read ranges.

The following examples describe some exemplary constructions of variousembodiments of the signage articles described in the presentapplication. The following examples also report some of the performanceresults of the signage article constructions.

EXAMPLE 1

A signage article was made using a plastic substrate onto which wasadhered or affixed non-metallized prismatic reflective sheeting. RFIDtags (915 MHz) manufactured by Transcore Co. were affixed to one side ofeach of a variety of plastic substrates having thicknesses ranging from1/10 inch (2.54 mm) to ½ inch (12.7 mm). Retroreflective Diamond Grade™nonmetallic sheeting manufactured by 3M Company of St. Paul, Minn. wasapplied to the other side of each of the plastic substrates. The RFIDtag on each of the plastic substrates was successfully read with anIntermec handheld reader at a distance of approximately 30 feet.

EXAMPLE 2

A cutout roughly 0.5 inch (12.7 mm) greater in dimension on all sidesthan a 2 inch (5.08 cm) by 3 inch (7.62 cm) 915 MHz passive TranscoreRFID tag was cut into a 0.08 inch (2.03 mm) thick, 18 inch (45.7 cm) by24 inch (61 cm) aluminum sign substrate. One of the major surfaces ofthe sign substrate was covered with retroreflective Diamond Grade™sheeting manufactured by 3M Company of St. Paul, Minn. The RFID tag wasplaced into the cutout. A plastic plug having a thickness and shape thatapproximately matched the thickness and shape of the removed signsubstrate was placed into the cutout such that the plastic plug waspositioned adjacent to the RFID tag. Thus the plastic plug wassubstantially similar in size and shape to a size and shape of thecutout. A piece of non-metallic tape was placed on the backside of thesign substrate such that it completely covered the cutout and waspositioned adjacent to the plastic plug. The RFID tag was successfullyread with an Intermec handheld reader at a distance of approximately 30feet, a distance comparable to the nonconductive RFID signage articlesas described in Example 1.

Additionally, the inventors of the present application determined thatby carefully controlling or selecting the location of the cutout,opening, or aperture and/or the RFID tag within the cutout, opening, oraperture on the signage, one can control or tailor the radiation patternof the RFID tag. The term “location” includes, for example, the positionof the cutout, opening, or aperture on the signage as well as theplacement of the RFID tag within the signage or the RFID integratedcircuit across the cutout, opening, or aperture. Controlling theradiation pattern can facilitate direction of radiation from the signagetoward the roadway and can reduce the amount of radiation that isdirected in an unwanted or unproductive direction.

FIG. 7 depicts the location of each of the various cutouts that were cutfrom various plates for testing the effects of the cutout location.Cutouts 1 through 5 were used to investigate the effect of moving theminor (shorter) edge of the cutout toward the vertical edge of the metalsheet. Cutouts A through I were used to investigate the effect of movingthe major (longer) edge of the cutout toward the horizontal edge of themetal sheet. Cutout A is the same as cutout 1.

FIGS. 8A, 8B, 8C, and 8D pictorially show how the thirteen cutoutlocations shown in FIG. 7 were implemented in four separate pieces ofmetal plate. The plate of FIG. 8A includes cutout locations A/1, E, andH. The plate of FIG. 8B includes cutout locations D, 2, and G. The plateof FIG. 8C includes cutout locations C, 3, 5, and F. The plate of FIG.8D includes cutout locations 4, B, and I.

Each cutout was 4.5 inches (11.4 cm) long and 1 inch (2.54 cm) high. Themetal plate was a 12 inches (30.48 cm) by 12 inches (30.48 cm) metalplate cut from ⅛ inch (0.317 cm) aluminum stock. During testing, a 915MHz RFID tag ((a 4 inch long (10.16 cm) and ½ inch (1.27 cm) highSquiggle™ RFID tag manufactured by Alien Co.)) was placed in the centerof one of the cutouts located in a single metal plate. The unusedcutouts on each metal plate being tested were covered with copper tapeduring testing of a particular cutout. When cutouts 1 through 5 weretested, the metal plate was rotated about the y-axis (as shown in FIG.7) with 90 degrees being normal to the plate and 0 degrees being off theleft side of the plate. When cutouts A through I were tested, the metalplate was rotated about the x-axis (as shown in FIG. 7) with 90 degreesbeing normal to the plate and 0 degrees being off the top edge of theplate. The read performance of each plate was measured by placing anattenuator in between an RFID reader and a reader antenna. Theattenuation was increased in 1 dB increments until the tag could nolonger be read. This testing was performed in an anechoic chamber at afixed distance between the circularly polarized reader antenna and themetal plate including the RFID tag.

FIG. 9 shows the effective radiation pattern of an RFID tag positionedin cutouts A, C, E, G, and I. The effective radiation pattern of an RFIDtag positioned in cutouts B, D, F, and H was not tested. FIG. 9 showsthat the radiation pattern for an RFID tag in a cutout near the centerof the plate (e.g., cutouts A and C) is nearly symmetrical about 90degrees (nearly normal to the plate). As the cutout location is movedtoward the top of the plate (e.g., cutouts E and G), the radiationpattern shifts away from 90 degrees towards 120 degrees. For cutout I,the radiation pattern is maximum at 15 degrees, nearly off the edge ofthe plate.

These results allowed the present inventors to conclude that bycontrolling the placement of the cutout and RFID tag, one could create asign including RFID functionality that was tailored for its intendedplacement and use. In other words, by controlling the spacing betweenthe edge of the metal plate and the major side/surface of the cutout,the radiation pattern could be controlled. For example, a sign could becreated that was best read from a location at an off-angle by placingthe cutout and RFID tag toward the edge of the metal plate, e.g., at thecutout I location. Alternatively, a sign could be created that was bestread from a location normal to the sign by placing the cutout and RFIDtag toward the middle of the metal plate, e.g., at the cutout A/1location.

FIG. 10 shows the effective radiation pattern of cutouts 1 through 5.All of these cutouts resulted in a maximum radiation that is normal tothe plate at about 90 degrees. Thus, controlling the location betweenthe minor side of the cutout and the plate edge did not appear to be aneffective method of controlling the effective radiation pattern.

As was described above, the performance results shown in FIGS. 9 and 10are based on a metal sign in which the RFID tag is centered in thecutout (centered along the x-axis and the y-axis and the z-axis of thecutout). Based on the fact that the metal plate was ⅛ inch (0.317 cm)thick, this meant that the RFID tag was positioned at approximately 1/16inch (0.15 cm) from each major surface (the front and back surfaces) ofthe metal plate.

FIG. 11 shows the effect of having the RFID tag centered along thex-axis and y-axis of the cutout (at position A/1) but at differentlocations along the z-axis of the cutout. In other words, the RFID tagwas placed at different thicknesses within cutout A/1. The results shownin FIG. 11 allowed the present inventors to conclude that when the tagis centered within the thickness of cutout A/1 (or along the z-axis),the tag performs about 6 dB better from the front of the plate than ifthe tag is flush with the back of the metal plate. This effect can beused to control the radiation pattern. For example, if the tag iscentered within the cutout (along the z-axis), then the tag will radiatefairly well normal to both the front and the back of the metal plate (orsign). If the tag is flush with the front of the metal plate (or sign),the tag will radiate better off of the front of the plate than off ofthe back of the plate. If the tag is flush with the back of the metalplate (or sign), the tag will radiate better off of the back of theplate than off of the front of the plate.

EXAMPLE 3

The following example was modeled to determine the effect of changingthe y dimension of the cutout with the tag centered in the x and ydimensions with respect to the metal plate and located flush with thefront of the plate (along the z dimension). A 4.5 inches (11.43 cm) longand 3.5 inches (8.89 cm) high cutout in position A (as described above)was modeled in a metal plate measuring 12 inches (30.48 cm) by 12 inches(30.48 cm) and made of ⅛ inch (0.137 cm) aluminum stock. The cutout usedfor this model was identical to cutout A except that the y dimension wasmade to be 3.5 inches (8.89 cm) by moving the upper edge of the cutoutupward. The results are shown in FIG. 12. FIG. 12 shows that when the ydimension of the cutout is enlarged, the radiation pattern changes witha reduction in directivity. Thus the size of the cutout can be used tochange or optimize the radiation pattern.

EXAMPLE 4

The following example was modeled to determine the effect of changingthe metal plate size while keeping the cutout with the tag centered inthe x and y dimensions with respect to the metal plate and located flushwith the front of the plate (along the z dimension). Two separate metalplates of differing sizes were modeled. Both plates were metal platesmade of ⅛ inch (0.137 cm) aluminum stock and the cutout in each platemeasured 4.5 inches (11.43 cm) long and 1 inch (2.54 cm) high. The sizeof Plate A was 12 inches (30.48 cm) by 12 inches (30.48 cm), and thesize of Plate B was 6 inches (15.24 cm) by 6 inches (15.24 cm). Themodeling results are shown in FIG. 13. FIG. 13 shows that when the ydimension of the cutout is enlarged, the radiation pattern changes witha reduction in directivity. Thus the size of the cutout can be used tochange or optimize the radiation pattern.

EXAMPLE 5

The following example was modeled to determine the effect of changingthe metal plate shape while keeping the cutout with the tag centered inthe x and y dimensions with respect to the metal plate and located flushwith the front of the plate (along the z dimension). Two separate metalplates of differing shapes were modeled. Both plates were metal platesmade of ⅛ inch (0.137 cm) aluminum stock and the cutout in each platemeasured 4.5 inches (11.43 cm) long and 1 inch high (2.54 cm). The plateshapes modeled were a square and a triangle. The square plate measured12 inches (30.48 cm) by 12 inches (30.48 cm). The triangle plate had abase of 12 inches (30.48 cm) and sides of 13.2 inches (33.53 cm), i.e.,a height of 12 inches (30.48 cm). The modeling results are shown in FIG.14. FIG. 14 shows that when the shape of the metal plate is changed, theradiation pattern changes. Thus the shape of the metal plate can be usedto change or optimize the radiation pattern.

The same effects in the radiation patterns for cutout size, signagesize, and signage shape are expected to apply for signage developed withthe electrically conductive component being a sheeting applied to thesubstrate.

II. Signage Constructions Including a Slot Antenna and an ElectricallyConductive Substrate

Another exemplary embodiment of the present invention includes forming acutout, opening, slot, or aperture in an electrically conductivesubstrate such that the conductive substrate operates as an antenna foran RFID integrated circuit that is coupled to the substrate. Thiscutout, opening, or aperture creates what can be referred to as a “slotantenna,” which has radiation pattern properties similar to that of adipole antenna. When an RFID chip is attached to the electricallyconductive substrate, such as, for example, a metal street sign, theslot antenna interfaces with the RFID chip. Thus RFID functionality canbe associated with an electrically conductive signage by (1) creating acutout, opening, slot or aperture in an electrically conductivesubstrate to form a slot antenna; and (2) attaching an RFID chip to theelectrically conductive substrate such that the slot antenna formedwithin the signage functions as the RFID antenna and such that theresulting electrically conductive substrate has RFID functionality. Thepresent inventors recognized that the relatively large form factor ofthe signage can create a very efficient radiator if the aperture isdesigned and driven appropriately.

Silicon integrated circuit chips generally have a low resistance and alarge negative reactance. There are two methods of achieving powertransfer; design a matching network to transform the chip impedance tothe antenna impedance or design the RFID antenna to directly match thechip impedance. In many RFID applications, the space constraints dictateusing the latter approach. However, in signage applications, the formfactor can be significantly larger than in many other applications,allowing both options to be viable. Consequently, the present inventorsinvestigated both the integrated matching network approach and thedirect antenna matching approach, both of which are included in thepresent application.

A. Slot Antenna with an Integrated Matching Network

In this implementation of signage articles including a slot antenna, theslot antenna may be designed independent of the chip impedance and mayuse a matching network to transform the impedance between the RFID chipand the antenna. One advantage of this design is that the antenna designremains constant. However, the matching network components will have tobe adjusted based on the specific RFID chip that is implemented in thedesign.

One exemplary implementation of this embodiment is shown in FIGS. 5 aand 5 b. FIG. 5 a is a schematic view of a metal speed limit sign 50including a radio frequency-responsive element 52. FIG. 5 b is anexploded view of the portion of sign 50 including radiofrequency-responsive element 52. In FIG. 5 b, sign 50 includes arectangular cutout, opening, or aperture 54 having a top major surface56 and a bottom major surface 58. Aperture 54 is made in a major surfaceof a substrate (the substrate may be as is described above with respectto the “cutout” applications of the present application). An RFID tag orchip 82 is attached to a conductive interconnect 84 that is attached toa strap 86 (sometimes called an interposer) having a first major side68, a second major side 70, a first minor side 72, and a second minorside 74. First minor side 72 of strap 86 is positioned adjacent to topmajor surface 56 of aperture 54 and second minor side 74 of strap 86 ispositioned adjacent to bottom major surface 58 of aperture 54.Consequently, strap 86 is attached to sign 50 such that strap 86 ispositioned vertically within the horizontal aperture 54 and such thatthe RFID chip 82 is positioned within or adjacent to aperture 54. Strap86 may have an electrically direct contact to the conductive signage, ormay be capacitively coupled to the signage. Strap 86 is preferably aflexible substrate that provides electrical and physical connection ofthe RFID chip to the signage. Consequently, strap 86 physically andelectrically ties the metal of sign 50 to the RFID chip 82. Becausestrap 86 can be placed directly across aperture 54 and aperture 54 actsas a slot antenna, there is no need to fabricate a separate tag antenna.

An RFID reader (not shown) induces signals across the aperture which thestrap routes to the RFID chip. It is important that there be no metalinside or covering the aperture. So, non-metallic reflective or otherdielectric material sheeting could be used in this area, such as, forexample, Diamond Grade™ reflective sheeting manufactured by 3M Companyof St. Paul, Minn. Alternatively, a companion aperture could be cut intometallic reflective sheeting, as was described above in greater detail.

In this implementation, strap 86, interconnect 84, and RFID chip 82 arepreferably relatively flush with sign 50. Alternatively, they may bepositioned within the thickness of sign 50. This facilitates thestacking of multiple signs on top of one another without causing injuryor damage to the RFID chip attached to or associated with the sign.Strap 86 preferably has a minimal thickness, for example, a thicknessbetween about 0.5 mil and about 5 mils.

Those of skill in the art will appreciate that many changes can be madeto the embodiment shown in FIGS. 5 a and 5 b without departing from thespirit of the concept. For example, although strap 86 is shown asattached to the front face of sign 50 in FIGS. 5 a and 5 b, strap 86 canalso be attached to the rear face of sign 50 (not shown) or within thethickness of sign 50. Also, aperture 54 need not be rectangular inshape; aperture 54 can be of any desirable shape (e.g., tapered slotsand annular rings) and can be placed in any desirable location on sign50 including the use of multiple slots or arrays of slots. Additionally,the signage may include an array of apertures, which may increase theantenna directivity and the read-range as well as modify the radiationpattern. Further, sign 50 need not be a rectangular speed limit sign,but can be any shape or form of signage. Additionally, strap 86 can beshaped differently (e.g., wider, shorter, longer, thinner) than shown inFIGS. 5 a and 5 b.

The following example describes one exemplary construction of a signagedescribed above as well as reporting some of the performance results ofthe signage construction.

EXAMPLE 6

A 50-ohm slot antenna was designed for operation at 915 MHz usingstandard slot design equations. The antenna was modeled using CSTMicrowave Studio™ and measured on a network analyzer. A schematic viewof the slot design is shown in FIG. 15. A single side metallized FR4 PCboard 402 measuring 6 inches (15.24 cm) by 12 inches (30.48 cm) was usedas a substrate (the approximate size of a standard United States licenseplate). Slot antenna 400 measured 8 inches (20.32 cm) by 0.4 inch (1.02cm) and was created by mechanically etching the copper from the PC boardsurface.

FIGS. 16 a and 16 b show the modeled radiation profile of the slotantenna of FIG. 15. FIGS. 16 a and 16 b show that the slot antenna ofFIG. 15 focuses radiation with a directivity of 6 dBi. For comparison, atypical RFID antenna (i.e., a dipole antenna) typically has adirectivity of approximately 2 dBi.

In at least some embodiments, using the metal of the electricallyconductive sign facilitates the readability of the RFID chip. In theseembodiments, the electrically conductive sheeting and/or the metalsubstrate of certain types of signage acts as the RFID antennae. Thusthe metal signage assists in the performance of the RFID chip ratherthan hindering its performance. Consequently, the read-range performanceis enhanced in at least some embodiments.

When additional control of the radiation pattern is desired (i.e.,control of nulls and/or beam lobes), it can be useful to includeadditional slots in the conductive signage to act as additional beamforming elements. These slots could be used as passive antenna elementsto create a beam forming array, such as in a yagi or other multi-elementantenna array design. For example, the elements can be arranged in anarray of elements to increase the antenna directivity and possiblycontrol the angle of maximum radiation. An array of elements includes,for example, multiple slots that are electrically coordinated by spacingand/or phasing to produce a desired radiation pattern.

The present inventors recognized that changing the shape or size of theplate can result in changes in the direction of the radiation patterns.For example, the diffraction from the edges of the plate can cause theradiation pattern to change as the size and shape of the plate changes.In some embodiments, it may be desirable to create an omni directionalantenna pattern, while in other instances it may be preferred to have amore directional antenna pattern. For example, it may be desirable tohave roadside signage with an antenna pattern directed toward the roadtraffic that it is intended to address. Modeling was used to investigatehow the pattern of a slot antenna can be impacted by the size of themetal plate in which it is implemented. Various exemplary plates weremodeled and the results are shown in the Example below.

EXAMPLE 7

A 900 MHz 50 ohm slot antenna was modeled such that it was centered inthree plates of various sizes. Plate #1 measured 0.5 foot by 1 foot;Plate #2 measured 2 feet by 2 feet; and Plate #3 measured 4 feet by 4feet. The resultant antenna patterns are shown in FIGS. 16-19.Specifically, the antenna pattern for Plate #1 is shown in FIGS. 16 aand 16 b (where FIG. 16 a is the front view and FIG. 16 b is the sideview); the antenna pattern for Plate #2 is shown in FIGS. 17 a and 17 b(where FIG. 17 a is the front view and FIG. 17 b is the side view); andthe antenna pattern for Plate #3 is shown in FIGS. 18 a and 18 b (whereFIG. 18 a is the front view and FIG. 18 b is the side view). Finally,FIG. 19 facilitates a comparison between FIGS. 16-17.

FIGS. 16-19 show that increasing the plate size increases the radiationoff of the broadside of the plate and shows that increase in plate sizeincreases ripple in the radiation pattern. FIG. 19 indicates that of thethree plate sizes modeled, the slot in the 0.5 foot by 1 foot platewould produce the greatest read-range normal to the plate, but the worstread-range at angles far from the normal. FIG. 19 also indicates that atapproximately 60 degrees from the normal, of the plates modeled, thelargest plate produces the best read-range.

EXAMPLE 8

A 50-ohm slot antenna was designed for operation at 915 MHz usingstandard slot design equations. A single side metallized FR4 PC boardmeasuring 6 inches (15.24 cm) by 12 inches (30.48 cm) was used as asubstrate. The 8 inches (20.32 cm) by 0.4 inch (1.02 cm) slot antennawas created by mechanically etching the copper from the PC boardsurface. To flush mount and integrate the matching circuit, a hole wasmilled through the circuit board substrate in the center of the slotantenna and a small matching circuit board was inserted, as is shown inFIGS. 25 a and 25 b. The circuit board includes the components of thematching circuit. Reference numeral 450 indicates the RFID integratedcircuit. While those of skill in the art will recognize that variouscircuit designs may be implemented including differing layouts andelements, the circuit design that was used in this example is shown inFIG. 26. As is shown in FIG. 26, the matching circuit that was used inthis example includes multiple capacitors to provide a DC block. Thiswill not be necessary in many applications, some of which can becompleted, for example, with an inductor and a capacitor. The values ofthe individual components of the matching circuit are not shown, butthose of skill in the art will recognize that these values will varywith different chip models and manufacturers. The formation of aseparate matching circuit board permits the optimization of the matchingcircuit independent of the slot antenna. The matching circuit and slotantenna are then integrated in a separate step.

Those skilled in the art will realize that the matching network could berealized with stub tuning, which could be very thin if implemented usingtransmission line structures. The term “transmission line structures” ismeant to include, but not be limited to, stripline, microstrip, andcoplanar waveguides. Also, the matching network can be placedasymmetrically in the slot to provide the appropriate impedance matchingcharacteristics.

B. Slot Antenna Directly Matched to the Chip

In this implementation of signages including a slot antenna, a slotdesign was formed that directly matches the chip impedance. Oneadvantage of this design is that the matching network is eliminated,thus eliminating the complications that may arise as a result ofintegrating the matching network into the feed. However, one potentialdisadvantage of this design is that the antenna must be redesigned if achip with a different impedance is implemented.

To match the slot directly to the chip, the aperture dimensions must beformulated to match the chip impedance. Consequently, the dimensions ofthe slot or aperture will vary based on the specific chip modelimplemented. The present inventors used modeling techniques to determinethe optimal slot dimensions for a commercially available chipmanufactured by Philips. The slot antenna was etched onto a single sideof a metallized FR4 PC board measuring 6 inches (15.24 cm) by 12 inches(30.48 cm), as is described in Example 6. Formed in the center of themetal plate was a 3 inch (7.62 cm) long by 0.8 inch (2.03 cm) highaperture or slot. To ensure proper operation of the specific RFIDintegrated circuit used, DC blocking capacitors were used, as isdescribed above. Those of skill in the art will appreciate that DCblocking capacitors may not be required in all implementations of thistype of slot antenna. Read-range tests of a signage formed as describedabove were performed in an anechoic chamber. External electromagneticinterference was prevented or minimized by shielding the interior of thechamber with copper sheeting. Electromagnetic reflections within thechamber were prevented or minimized by absorption cones. The readerantenna was placed at one end of the chamber and the signage includingan RFID tag that was being tested was placed at the other end of thechamber. The overall distance between the reader antenna and the slotantenna was approximately five feet. The power of the reader was set to31 dBm. The cable loss was assumed to be 1 dB, therefore the resultingpower output was 30 dBm. To determine the read-range, the reader powerwas attenuated in increments of 1 dB until the tag was no longerreadable. For all measurements, the height of the slot tag was alignedwith the reader antenna. The results were as shown in Table I below.

TABLE I Results of Read-Range Testing for Matching Network and DirectlyMatched Slots. Design Attenuation Setting Calculated Read-Range MatchingNetwork 9 dB 14.2 ft. Directly Matched Slots 9 dB 14.2 ft.

Table I shows that the read-range results for the matching network andthe directly matched slots were the same.

The present inventors also recognized that changing the shape, size, andlocation of the cutout, opening, slot, or aperture can result in changesin the direction of the radiation patterns and changes in the antennaimpedance. When designing an RFID enabled sign, varying the size of themetallic road sign may not be an option. However, controlling thelocation of the slot antenna may be a more viable option. To furtherinvestigate the opportunity to control the slot antenna pattern, variouslocations of the slot within a plate of a fixed size was modeled. Thepositions of the slot locations that were evaluated are show in FIGS. 20and 21. The antenna patterns cut were formed in the same plane asdescribed in Example 7. The results of the modeling are shown in FIGS.22 & 23. The results shown in FIG. 22 relate to the slot antennalocations shown in FIG. 20, and the results shown in FIG. 24 relate tothe slot antenna locations shown in FIG. 21. FIG. 23 shows the modelingcoordinate system (x, y, z, theta, and phi).

FIGS. 20-24 allowed the present inventors to conclude that moving thebroad edge of the slot toward the top edge of the plate (slots 1 and 2)directs the radiation pattern toward the bottom edge of the plate. In aneffort to design an antenna pattern where radiation is directedprimarily off the edge of the metal plate, the slot antenna maypreferably be positioned so as to break through the periphery of themetal plate as is shown in FIG. 21. This positioning can produce apreferred direction of radiation off the edge of the metal sign.

All of the embodiments described above create a signage identificationsystem including an electrically conductive signage and an RFID element.The signage may, for example, be used to create electronic signs torelay information to drivers electronically. Alternatively, the signagemay be used to track signage during manufacture or to track signage thatneeds to be placed, replaced, repaired, or changed. Also, it will beappreciated that the above embodiments and implementations may include asurface acoustic wave (SAW) RFID tag rather than a traditional RFID tag.

The signage may include an optical surface where light incident on theoptical surface is reflected or retroreflected from the signage backtowards the light source. The radio frequency-responsive elementpreferably has information storage and transmission capability. Theradio frequency-responsive element is preferably adapted to enable aninterrogation system to obtain information from the element. Theradiation pattern can be selected by design of the size of the signageand/or aperture, opening, slot, or cutout.

The RFID tags and/or chips described above may be attached or affixed tothe electrically conductive signage when it is manufactured or may beapplied at a point later in the signage's life.

Those having skill in the art will appreciate that many changes may bemade to the details of the above-described embodiments without departingfrom the underlying principles thereof. The scope of the presentapplication should, therefore, be determined only by the followingclaims.

1. An electrically conductive, radio frequency identification(RFID)-enabled signage article, comprising: a slot antenna including anelectrically conductive element having an opening; and an RFIDintegrated circuit coupled to the electrically conductive element,wherein the slot antenna operates as an RFID antenna; wherein theelectrically conductive element includes at least one of an electricallyconductive substrate or an electrically conductive sheeting.
 2. Thesignage article of claim 1, wherein the electrically conductive elementis a conductive substrate positioned adjacent to optically activesheeting.
 3. The signage article of claim 1, wherein the electricallyconductive element is a non-conductive substrate positioned adjacent toconductive optically active sheeting.
 4. The signage article of claim 1,wherein the electrically conductive sheeting is retroreflectivesheeting.
 5. The signage article of claim 1, further comprising: amatching network electrically coupled in series between the RFIDintegrated circuit and the electrically conductive element thattransforms the impedance between the RFID integrated circuit and theelectrically conductive element.
 6. The signage article of claim 5,wherein the matching network includes a transmission line structure. 7.The signage article of claim 1, including an electrically conductivesubstrate and an impedance of the antenna substantially matches animpedance of the RFID integrated circuit, and wherein the electricallyconductive substrate is directly electrically coupled to the RFIDintegrated circuit.
 8. The signage article of claim 1, furthercomprising a strap positioned to span at least a portion of the openingof the slot antenna and to electrically couple the RFID integratedcircuit to the electrically conductive element.
 9. The signage articleof claim 8, wherein the RFID integrated circuit is attached to thestrap.
 10. The signage article of claim 8, wherein the strap and theRFID integrated circuit are substantially flush with a major surface ofthe electrically conductive element.
 11. The signage article of claim 8,wherein the strap is positioned between a first major surface and asecond major surface of the electrically conductive element such thatthe strap is within a thickness of the electrically conductive element.12. The signage article of claim 8, wherein the strap has a thickness ofat least 0.5 mil (0.0005 inch).
 13. The signage article of claim 8,wherein the strap is in direct electrical contact with at least aportion of the electrically conductive element.
 14. The signage articleof claim 8, wherein the strap is capacitively coupled to at least aportion of the electrically conductive element.
 15. The signage articleof claim 1, wherein the opening forms a rectangular slot.
 16. Thesignage article of claim 15, wherein the rectangular slot has a lengthand a width and the length of the rectangular slot is at least ten timesgreater than the width of the rectangular slot.
 17. The signage articleof claim 1, wherein the signage article comprises at least one of atraffic control material; a vehicle marking; a roadway marking; aretroreflective garment; an indoor/outdoor labeling product; a frangiblesecurity sticker; a product authentication material; a store displaypackage; a document; an inventory labeling and control product; anidentification tag; an identification label; an identification system; alicense plate; or a road sign.
 18. The signage article of claim 1,wherein the opening forms an annular ring.
 19. The signage article ofclaim 1, wherein the opening is one slot in an array of slots.
 20. Thesignage article of claim 1, wherein the RFID integrated circuit is anRFID integrated circuit utilized in a surface acoustic wave (SAW) RFIDtag.
 21. The signage article of claim 1, wherein the RFID integratedcircuit stores information related to the signage article.
 22. Thesignage article of claim 21, wherein the RFID integrated circuit storesthe inspection history of the signage article.
 23. A method of formingan electrically conductive, RFID-enabled signage article, comprising:selecting an electrically conductive element having a length and awidth; forming an opening in at least a portion of the electricallyconductive element; and coupling a radio frequency identification (RFID)integrated circuit to the electrically conductive element; wherein theselecting a substrate having a length and a width is based on a desiredradiation direction/pattern of the electrically conductive, RFID-enabledsignage article that is formed when the RFID integrated circuit iscoupled with the electrically conductive element.
 24. The method ofclaim 23, wherein the electrically conductive element is a conductivesubstrate positioned adjacent to optically active sheeting.
 25. Themethod of claim 23, wherein the electrically conductive element is anon-conductive substrate positioned adjacent to conductive opticallyactive sheeting.
 26. The method of claim 23, wherein the coupling aradio frequency identification (RFID) integrated circuit to theelectrically conductive element involves attaching the RFID integratedcircuit to a strap and coupling the strap to the electrically conductiveelement.
 27. The method of claim 23, further comprising: selecting amatching network to transform the impedance between the RFID integratedcircuit and the electrically conductive element; and placing thematching network between the RFID integrated circuit and theelectrically conductive element.
 28. The method of claim 23, wherein theRFID integrated circuit is positioned asymmetrically off-center withinthe opening in the signage article to effect a desired direction ofradiation.
 29. The method of claim 23, wherein the RFID integratedcircuit is positioned symmetrically and centered within the opening inthe signage article to effect a desired direction of radiation.
 30. Amethod, comprising: selecting a location on an electrically conductiveelement; forming an opening in at least a portion of the electricallyconductive element at the selected location; and coupling a radiofrequency identification (RFID) integrated circuit to at least a portionof the electrically conductive element; wherein the selecting a locationon the electrically conductive element is based on a desired radiationdirection/pattern of a electrically conductive, RFID-enabled signagearticle that is formed when the RFID integrated circuit is coupled withthe electrically conductive element.